The present invention relates to an optical disk apparatus for optically reproducing information from an optical disk by focusing a light beam onto the optical disk upon which information is recorded, and more particularly to an optical disk apparatus for performing reproduction while moving a beam spot over an optical disk, the beam being shifted in position away from the center line of the prepit arrays in the radial direction relative to the center of the optical disk.
A DVD-RAM (Digital Versatile Disk-Random Access Memory) has been developed as one of several large-capacity rewritable optical disks, and the details of a standard for this DVD-RAM are described in a book named "DVD-Specifications for Rewritable Disc version 1.0". The book prescribes address information and the like must be pre-recorded in a header field as prepits. "Prepits" are information-containing pit arrays encoded with sector address information, permanently formed into the surface of the disk, and preceding the recording fields. The book also prescribes employing in recording fields both grooves and the lands that separate the grooves, which are created on the disk beforehand, and recording user information, such as image data, audio data or the like on both the lands and the grooves. Accordingly, it is required that the prepits must be playbacked from extensions of both a land track and a groove track in the same way.
FIG. 1 is a diagram showing an example disk format for this current type of DVD-RAM. The recording fields 42 for use in recording various kinds of the user information (such as image data and sound data, etc.) are formed by the lands 54 and the grooves 52, and the header field 44 in which the address information has been pre-recorded are formed by arrays of pits formed as the prepits 8. The center lines C1, D1 of these prepit arrays 8 are shifted radially (with respect to the center of the optical disk 10) in a manner such that the center lines C1, D1 are placed on extension lines of the boundaries between adjoining lands 54 and grooves 52, as is shown in FIG. 1.
Therefore, a beam spot 6 generated by focusing a light beam onto the optical disk 10 scans along the center lines of the lands 54 and the grooves 52 in the recording field 42, and the beam spot 6 also scans the prepit arrays and in the header field 44, shifted from prepit array center lines by a predetermined distance, a pitch tp/2 which is one-half of the track pitch tp.
The light beam reflected from the optical disk 10 is focused upon a photo detector which performs information reproduction from the land track and the groove track as well as from the prepit arrays. The photo detector has a light-receiving surface which is divided into two light-receiving segments by a divisional line extending tangentially parallel to the tracks of the optical disk 10, and the photo detector is disposed such that the divisional line passes approximately through the center of a zero-order diffracted beam of the reflected light beam. With regard to the information reproduction from the prepit arrays, a "push-pull" signal, which corresponds to the difference between output signals derived from the split light-receiving segments of the photo detector, is generated as the information playback signal. This playback signal is then processed by a signal processing circuit that then presents the information recorded within the prepit arrays as well as that recorded within the land track and the groove track.
To meet demands that such rewritable optical disks should achieve greater storage capacity, a high-density DVD-RAM that can record information at even higher areal densities than have been achieved by the current DVD-RAMs is under development. In this high-density DVD-RAM, to increase its areal recording density, the track pitch must be made narrower than that of current DVD-RAMs. Where the reproduction of information from a prepit array on a high-density DVD-RAM is carried out using a light beam by letting the beam spot be offset by a predetermined distance from the center line of the prepit array, as is done with the current DVD-RAM, this narrowing of the track pitch causes the distance between the center point of the beam spot and the center line of the prepit array to decrease so that the resulting distance is less than that employed in the current DVD-RAMs.
When deriving the push-pull signal from such a high-density DVD-RAM in the same manner as when doing so from the current DVD-RAM, it has been found the less the distance between the center point of the beam spot and the center line of the prepit array, the less the amplitude of the push-pull signal, and when the center of the beam spot scans a location just overlying the center line of the pre-pit array, the resulting amplitude of the push-pull signal becomes essentially zero. Therefore, when reproducing high density DVD-RAMs with the track pitch narrowed, the push-pull signal decreases in amplitude to the point where there can be difficulty in the reproduction of the information and, possibly, errors.
Accordingly, it is an object of the present invention to solve the above-mentioned problem by providing an optical disk apparatus capable of producing an information reproduction signal of greater signal amplitude even when used with a plurality of types of optical disks that differ in track pitch and in recording density, when reproducing information by using a light beam to scan certain lines spaced apart by a predefined distance from the center of a mark array having an optical phase difference relative to non-mark regions, such as those where prepit arrays are located.